(1) A molecular replacement solution of the crystal structure of the Fab of CC49, a murine monoclonal antibody against solid adenocarcinoma, has been obtained and is being refined. (2) Small crystals of a single-chain construct containing the VL and VH of CC49 fused to human IgG1 CH2 and CH3 have been obtained. (3) Models of the IgE-binding, lectin-homology domains of the human and murine low-affinity IgE receptors (CD23) have been built. (4) The binding of various viral and self peptides to the murine class I MHC antigen, H-2Dd, has been modelled. (5) A molecular replacement solution of the crystal structure of unliganded HyHEL-10 Fab has been obtained and is being refined. (6) Crystallographic studies of several proteins have been initiated.